I suppose if you need to reach for it, that's fine, but why would you want a straight armed forehand just for a basic forehand?

Consistency; Its much easier for kids early on to use straight arm as its easier for the muscle to remember. Its actually easier to replicate and achieve consistency.

From a biomechanic point of view, the longer the extension the more power. Would you rather get hit with a short stick or a long stick? kenetic energy is transfered better if the arm is at full extension at contact.